Blood oxygen saturation is the relative amount of oxygenated hemoglobin in all of the hemoglobin present in the blood stream. This hemoglobin is packaged in biconcave discs of approximately 10 micrometers diameter which commonly occur with a density of approximately five million red blood cells per cubic millimeter. When radiant energy (e.g., light) is incident upon red blood cells, the red blood cells both reflect and absorb the incident radiant energy. Oxygenated and non-oxygenated hemoglobin reflect and absorb the radiant energy incident on the red blood cells differently. Thus, oxygen saturation may be determined by analyzing the radiant energy scattered back from the red blood cells.
An oxymetry sensor can use radiant energy of one, two or more different centered wavelengths to obtain measures of blood oxygen saturation. The oxymetry sensor emits radiant energy of a particular wavelength into the blood and measures the radiant energy scattered back from the blood. For a given (non-isobestic) wavelength, the intensity of light scattered back by the blood varies based on the relative concentrations of hemoglobin and oxyhemoglobin. Because the relationship between scattering and oxygen saturation is known, the oxygen saturation can be calculated from the measured light intensity. However, single wavelength oxymetry systems are highly susceptible to noise and can only be used to measure relative changes in oxygen saturation.
Many sources of noise have the same effect upon different wavelengths of light and thus by looking at relative light scattering instead of absolute light scattering, the effects of many sources of noise can be eliminated. Thus, multi-wavelength oxymetry systems are less susceptible to noise and can be used to generate absolute measures of oxygen saturation. However, one problem with multi-wavelength oximetry sensors is that the relationship between scattering and oxygen saturation is different for each different wavelength. Furthermore, the relationship between scattering and oxygen saturation is different for different wavelengths and different hematocrit. Thus, even in multi-wavelength oximetry systems, the accuracy of the oxymetry system can vary over the range of hematocrit and oxygen saturation conditions which the oxymetry system may encounter.
In view of the disadvantages of the state of the art with respect to oxygen saturation, it would be desirable to have a multi-wavelength oxymetry system that achieves as accurate a reading of oxygen saturation as possible over a large range of possible conditions.
It would also be desirable to have a multi-wavelength oxymetry system that compensates for changes in oxygen saturation and hematocrit to ensure that oxygen saturation is measured accurately over a large range of possible conditions.